Lidar sensor assembly calibration based on reference surface

ABSTRACT

A LIDAR system includes one or more LIDAR sensor assemblies, which may be mounted to a vehicle or other object. Each LIDAR sensor assembly includes a laser light source to emit laser light, and a light sensor to produce a light signal in response to sensing reflected light corresponding to reflection of the laser light emitted by the laser light source from a reference surface that is fixed in relation to the LIDAR sensor assembly. A controller of the LIDAR sensor assembly may calibrate the LIDAR sensor assembly based at least in part on a signal from the light sensor indicating detection of reflected light corresponding to reflection of a pulse of laser light reflected from the reference surface.

RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 15/487,363, filed on Apr. 13, 2017, the disclosureof which claims priority to U.S. Provisional Application No. 62/440,761,filed Dec. 30, 2016, both of which are incorporated herein by reference.

BACKGROUND

The term “LIDAR” refers to a technique for measuring distances tovisible objects by emitting light and measuring properties of thereflections of the light. A LIDAR system has a light emitter and a lightsensor. The light emitter may comprise a laser that directs highlyfocused light toward an object which then reflects the light back to thelight sensor. The light sensor may comprise a photodetector such as aphotomultiplier or avalanche photodiode (APD) that converts lightintensity to a corresponding electrical signal. Optical components suchas lenses may be used in the light transmission and reception paths tofocus light, depending on the particular nature of the LIDAR system.

A LIDAR system has signal processing components that analyze reflectedlight signals to determine the distances to surfaces from which theemitted laser light has been reflected. For example, the system maymeasure the “time of flight” of a light signal as it travels from thelaser, to the surface, and back to the light sensor. A distance is thencalculated based on the known speed of light. However, the accuracy ofthe distance measurement may depend on performance characteristics ofthe components of the LIDAR system (e.g., power sources, light sources,light sensors, etc.). Additionally, changes in environmental conditions,such as temperature and/or humidity, may impact distance measurementsover time.

BRIEF DESCRIPTION OF THE DRAWINGS

The detailed description is described with reference to the accompanyingfigures. In the figures, the left-most digit(s) of a reference numberidentifies the figure in which the reference number first appears. Theuse of the same reference numbers in different figures indicates similaror identical components or features.

FIG. 1 is a partial cut away view showing an example LIDAR sensorassembly that can be calibrated using a reference surface that is fixedrelative to the LIDAR sensor assembly.

FIG. 2 is a timing diagram of a pulse emitted by an example LIDAR sensorassembly and a received pulse corresponding to light reflected off of afixed reference surface.

FIG. 3A is a perspective view of an example LIDAR sensor assemblyincluding a stationary support structure including a fixed referencesurface, and with an outer housing omitted for clarity.

FIG. 3B is a simplified top view of the example LIDAR sensor assembly ofFIG. 3A, shown with a top support rib omitted for clarity.

FIG. 4A is a perspective view of the example LIDAR sensor assembly FIG.3A showing the outer housing including a ring lens.

FIG. 4B is a simplified cross sectional view of the example LIDAR sensorassembly of FIG. 4A, taken along line B-B of FIG. 4A.

FIG. 5 is a simplified cross sectional view of another example LIDARsensor assembly with an unobstructed 360-degree detection angle.

FIG. 6 is a top plan view of an example vehicle having multiple LIDARsensor assemblies mounted to the vehicle.

FIG. 7 is a side view of an example vehicle showing example mountingorientations for LIDAR sensor assemblies.

FIG. 8 is a flowchart illustrating an example method of calibrating aLIDAR sensor assembly using a reference surface that is fixed relativeto the LIDAR sensor assembly.

DETAILED DESCRIPTION

Typical LIDAR systems emit a light pulse and detect reflected lightcorresponding to the light pulse reflected off an object in theenvironment. The reflected light signals are then analyzed to determinethe distances from the LIDAR system to surfaces from which the emittedlaser light has been reflected. For example, the system may measure the“time of flight” (TOF) of a light signal as it travels from the laser,to the object, and back to the light sensor. A distance is thencalculated based on the measured time of flight and the known speed oflight. However, existing LIDAR systems do not do not take into accountperformance characteristics of the components of the LIDAR systems.

For example, these known techniques assume that the light source emitsthe light pulse substantially instantaneously when it is instructed tofire. However, in practice, the light pulse is not emittedinstantaneously. Instead, there is some latency inherent in thecomponents of the LIDAR system. Moreover, the light pulse may beGaussian in nature, ramping up over time to a peak before dropping backoff Thus, the actual time of flight of the light pulse is a time from apeak of the emitted light pulse to a peak of the return pulse. However,because a time corresponding to the peak of the emitted light pulse maynot be known, existing LIDAR systems use as a proxy the time at whichthe light source is instructed to fire. Thus, existing LIDAR systems donot account for inaccuracies in distance measurements inherently causedby performance characteristics and limitations of the components of theLIDAR systems. Furthermore, existing LIDAR systems do not account fordifferences in performance characteristics between similar components ofthe LIDAR system (e.g., differences in characteristics between multipledifferent light sources within the LIDAR system). Existing LIDAR systemsalso do not account for changes in performance characteristics overtime, such as changes caused by environmental conditions in which theLIDAR systems are operating.

This application describes techniques for calibrating a LIDAR systembased on a reference surface that is fixed at a known distance from aLIDAR sensor assembly. By using a fixed reference surface that is aknown distance from the LIDAR sensor assembly, the LIDAR sensor assemblyis able accurately measure a time of the peak of the emitted lightpulse. In some examples this can be done directly by calculating anexpected time of flight of a light pulse to the reference surface andback to the LIDAR sensor assembly. From this expected time of flight,the LIDAR sensor assembly can accurately determine a latency from theinstruction to fire the light source to the peak of the emitted lightpulse that is attributable to the performance characteristics of thecomponents of the LIDAR sensor assembly. In other examples, due to therelatively short distance to the reference surface (typically a fewcentimeters or less), the time of flight of the light pulse to thereference surface may be negligible, and the LIDAR sensor assembly maydetermine the latency to be a time from the signal to fire the lightsource to receipt of the peak return signal.

Regardless of the technique used to determine the latency, the LIDARsensor assembly can be calibrated to account for the latency, therebyimproving the accuracy of subsequent distance measurements. In the caseof multi-channel LIDAR systems, these calibration techniques can beperformed for each channel (e.g., light source and light sensorcombination) of the LIDAR sensor assembly. Moreover, in some examples,these techniques can be applied at runtime to account for changes inperformance characteristics over time, such as changes caused byenvironmental conditions in which the LIDAR sensor assembly isoperating. Still further, in some examples, the intensity of thereflected light returned from the reference surface may be measured andcompared to previous returns to detect changes in performance of thelight sources (e.g., determine degradation, burnout, or malfunction of alight source, storage capacity of a capacitor or other power source, orthe like).

In some examples, a LIDAR sensor assembly usable to implement thetechniques described herein includes a rotatable assembly including oneor more light sources, one or more light sensors, and associatedcircuitry mounted in a chassis that rotates about a vertical rotationalaxis to scan horizontally across a scene. During a rotation of thechassis, light pulses are emitted at different horizontal directions.The horizontal angle of light emission varies with the rotation of thechassis. In other examples, LIDAR sensor assemblies according to thisdisclosure may be mounted in different orientations (e.g., may rotateabout an axis other than a vertical axis such that the LIDAR sensorassembly scans in a path other than horizontal). In some examples, aview of the LIDAR sensor assembly may be limited or partially obstructedby an opaque object (e.g., by a stationary portion of the LIDAR sensorassembly, a vehicle to which the LIDAR sensor assembly is mounted,etc.). In that case, the LIDAR sensor assembly may be said to have a“limited detection angle” of less than 360 degrees. The obstruction mayinclude a reference surface that is fixed relative to an axis ofrotation of the rotatable assembly. Thus, the reference surface ispositioned at a known, fixed distance from the light sources and lightsensors of the LIDAR sensor assembly and may be used to calibrate theLIDAR sensor assembly. In other examples, the LIDAR sensor assembly mayhave an unobstructed 360-degree detection angle.

In either case (limited detection angle or unobstructed detectionangle), the LIDAR sensor assembly may additionally or alternativelyinclude a substantially transparent surface (e.g., a cover or lenssurrounding the rotatable assembly). The substantially transparentsurface may be coupled to a stationary portion of the LIDAR sensorassembly and may be fixed at a known distance from the axis of rotationof the rotatable assembly. The substantially transparent surface mayreflect a portion of the light emitted by the light source and may,therefore, additionally or alternatively serve as a fixed referencesurface from which to calibrate the LIDAR sensor assembly.

In some examples, the calibration may be performed by a controller ofthe LIDAR sensor assembly as follows. The controller may cause the lightsource to emit a pulse of light toward the fixed reference surface. Thecontroller then receives a signal from the light sensor indicatingdetection of reflected light corresponding to reflection of the pulse oflight from the fixed reference surface. The controller may calibrate theLIDAR sensor assembly based at least in part on the signal indicatingdetection of the reflected light corresponding to reflection of thepulse of laser light from the fixed reference surface.

In this way, the LIDAR sensor assembly can be calibrated to account forlatency inherent in the performance characteristics of the lightsources, light sensors, and associated circuitry, thereby improving theaccuracy of subsequent distance measurements. In the case ofmulti-channel LIDAR systems, each channel (e.g., light source and lightsensor combination) may be calibrated. This calibration can be performedwhen the LIDAR system is turned on and/or periodically during use toaccount for changes in performance characteristics over time, such aschanges caused by environmental conditions in which the LIDAR sensorassembly is operating.

These and other aspects are described further below with reference tothe accompanying drawings. The drawings are merely exampleimplementations, and should not be construed to limit the scope of theclaims. For example, while the drawings depict a LIDAR sensor assemblyincluding a specific number of channels, the techniques described hereinare also applicable to LIDAR sensor assemblies using different numbersof channels. Also, while in some examples the LIDAR sensor assembly isdescribed as being mounted to a vehicle, in other examples LIDAR sensorassemblies according to this disclosure may be used in other scenarios,such as in a manufacturing line, in a security context, or the like.

Example LIDAR Sensor Assembly

FIG. 1 is a partial cutaway view of an example system including a LIDARsensor assembly 100. The LIDAR sensor assembly 100 includes a chassis102 that comprises multiple laser light source(s) 104(1)-104(N)(collectively referred to as “laser light sources 104) and one or morelight sensor(s) 106(1)-106(N) (collectively referred to as “lightsensors 106”), where N is any integer greater than or equal to 1. TheLIDAR sensor assembly 100 also includes control circuitry 108 configuredto control emission of light by the light sources and to receive andanalyze signals from the light sensors 106.

In some examples, the chassis 102 may include a partition 110 (shown astransparent for ease and clarity of installation) that forms acompartment on each of two lateral sides of the chassis 102. In FIG. 1,a sensor compartment 112 is shown on one side of the chassis 102 and anemitter compartment 114 is shown on the other side of the chassis 102.The sensor compartment 112 houses the light sensor(s) 106 and theemitter compartment 114 houses the laser light source(s) 104 while thepartition 110 may be opaque to prevent or limit light leakagetherebetween.

In the illustrated example, the chassis 102 also supports a first lens116 and a second lens 118, which may each be mounted so that theiroptical axes are oriented generally perpendicular to an outer surface ofthe chassis 102. The first lens 116 is generally above the emittercompartment 114 and forward of the laser light source(s) 104. In someexamples, one or more mirrors 120 are positioned within the chassis 102behind the first lens 116 and second lens 118 to redirect emitted andreceived light between horizontal and vertical directions. The chassis102 may be rotatable about an axis of rotation X, such that as thechassis 102 is rotated, the optical axes of the first lens 116 and thesecond lens 118 will scan horizontally across a scene including one ormore objects including an object 122.

In some examples, the LIDAR assembly 100 may include a plurality ofchannels by which a laser light sources 104 may emit light along aprecise direction so that the reflected light strikes a light sensorthat corresponds specifically to the laser light source 104. Forexample, laser light source 104(1) and light sensor 106(1) maycorrespond specifically to a first channel whereas laser light source104(N) and light sensor 106(N) may correspond specifically to an N-thchannel. The optical system of the LIDAR sensor assembly 100 is designedso that beams from light sources 104 at different physical positionswithin the LIDAR sensor assembly 100 are directed outwardly at differentangles in azimuth and elevation. Specifically, the first lens 116 isdesigned to direct light from the light sources 104 for at least some ofthe channels at different angles relative to the horizon. The first lens116 is designed so that the corresponding light sensor 106 of thechannel receives reflected light from the same direction.

The control circuitry 108 includes a controller 124 that implementscontrol and analysis logic. The controller 124 may be implemented inpart by an FPGA (field-programmable gate array), a microprocessor, a DSP(digital signal processor), or a combination of one or more of these andother control and processing elements, and may have associated memoryfor storing associated programs and data.

The controller 124 implements control and analysis logic for each of themultiple channels. To initiate a single distance measurement using asingle channel, the controller 124 generates a signal 126. The signal126 is received by a charge circuit 128, which determines an appropriatecharge duration (e.g., based on desired intensity, pulse width, etc.)and provides signal 130 to charge a capacitive driver 132 for thespecified charge duration. The capacitive driver 132 comprises a bank ofone or more capacitors to drive the light sources 104. The duration ofcharge determines the intensity of the light pulse emitted for by thelight source 104.

After charging for the specified duration, the controller 124 causes thecapacitive driver 132 to output an emitter drive signal 134 to therespective light source 104. The emitter drive signal 134 causes therespective light source (e.g., light source 104(1) in this example) tolight source 104 to emit one or more laser light pulses through thefirst lens 116 along an outward path 136 (shown by the dot- dash line).The burst is reflected by the object 122 in the scene, through the lenssecond 118, and to the light sensor 106 of the corresponding channel(e.g., light sensor 106(1) in this example) along a return path 138(shown by the double-dot-dash line).

Upon receipt of the reflected light along return path 138, the lightsensor 106(1) outputs a return signal 140 to an analog to digitalconverter (ADC) 142. The return signal 140 is generally of the sameshape as the emitter drive signal 134, although it may differ to someextent as a result of noise, interference, cross-talk between differentemitter/sensor pairs, interfering signals from other LIDAR devices,pulse stretching, and so forth. The return signal 140 will also bedelayed with respect to the emitter drive signal 134 by an amount oftime corresponding to the round-trip propagation time of the emittedlaser burst (i.e., the time of flight of the emitted burst).

The ADC 142 receives and digitizes the return signal 140 to produce adigitized return signal 144. The digitized return signal 144 is a streamof digital values indicating the magnitude and timing of the digitizedreturn signal 144 over time. In this example, the digitized returnsignal 144 is provided to a cross- correlator 146, which correlates aspecific digitized return signal 144 with the corresponding emitterdrive signal 134 and outputs a time of flight signal 148 indicative of atime shift from emission of the light pulse by the light source todetection of the reflection of the return of the light pulse at thelight sensor. In some configurations, the some or all of the functionsof the cross-correlator 146 may be performed by the controller 124. Oncea return signal is correlated or matched with an emitted signal, thecontroller 124 can then use the time of flight of the pulse of light incombination with the known speed of light to calculate a distance D tothe object 122. While the distance D is depicted in this figure as justa distance between the first lens 116 and the object 122, in practicethe distance D may take into account a total roundtrip distance of thelight path from the light source 104 to the light sensor 106 (i.e.,including the distances between the light sources 104 and light sensors106 and their respective lenses 116 and 118). The foregoing example isjust one of many techniques that may be used to recover the time offlight of the emitted pulse.

However, if, as in the case of FIG. 1, the distance D to the object 122is already known (i.e., if the object 122 is positioned at a known,fixed distance from the light source 104 and the light sensor 106), theobject 122 may serve as a reference surface and may be used to calibratethe LIDAR sensor assembly 100 as described further with reference toFIG. 3 below. That is, the time of flight signal 148 while the while theLIDAR assembly is aimed at the reference surface (i.e., object 122) canbe used as a “reference signal” against which to calibrate the LIDARsensor assembly 100. The reference signal (i.e., time of flight signal148 while the LIDAR assembly is aimed at the reference surface (i.e.,object 122)) may be captured uniquely for each channel in the LIDARsensor assembly 100, may be stored and used for multiple subsequentmeasurements, and may be updated over time to account for thermal driftand/or other variables. In some examples, the reference signal may beupdated at least once per revolution of the chassis. In other examples,the reference signal may be updated more or less frequently.Furthermore, in some examples, multiple readings may be performed andaveraged to create the reference signal.

Thus, by fixing the object 122 in a scan path of the optical axes of thefirst lens 116 and the second lens 118 at a known distance D from theLIDAR sensor assembly 100, the object 122 can be used as a referencesurface. In some examples, the object 122 may be part of the LIDARsensor assembly 100 (e.g., a support surface, part of the housing, alens, etc.), while in other examples, the object 122 may be part of asurrounding environment (e.g., a vehicle, machine, or other structure)which is fixed relative to the LIDAR sensor assembly 100.

Example Calibration of LIDAR Sensor Assembly

FIG. 2 is a timing diagram to illustrate calibration of a LIDAR sensorassembly such as that shown in FIG. 1. For ease of discussion, FIG. 2 isdescribed in the context of the LIDAR sensor assembly 100 of FIG. 1.However, the concepts illustrated in FIG. 2 are not limited toperformance by the LIDAR sensor assembly 100 and may be employed usingother systems and devices. Moreover, FIG. 2 depicts an example for asingle pulse on a single channel of a LIDAR system. However, in otherexamples, this technique may be performed for each channel of a LIDARsystem, and may be performed multiple times and/or using multiplepulses.

The timing diagram 200 includes a waveform 202 representing a pulseemitted by an example LIDAR sensor assembly and a waveform 204representing a received pulse corresponding to the emitted lightreflected off of a fixed reference surface. In some examples, theemitter drive signal 134 may be used as the waveform 202 representingthe emitted pulse, while the return signal 140 may be used as thewaveform 204 representing a received pulse corresponding to lightreflected off of a fixed reference surface.

As shown in FIG. 2, T₀ corresponds to a time at which the capacitivedriver 132 issues the emitter drive signal 134 to cause the light source104(1) to fire (i.e., the time at which the signal is transmitted).However, as discussed above, the light pulse is not emittedinstantaneously. Rather, when the emitter drive signal 134 is applied tothe light source 104(1), the light source 104(1) emits a Gaussian pulsethat ramps up over time to a peak before dropping back off T₁corresponds to the peak of the emitted light pulse, and T₂ correspondsto the peak of the return signal. The expected time of flight of thelight pulse is equal to a time between the peak of the emitted pulse T₁and the peak of the received pulse T₂. That is, expected time of flightequals T₂−T₁. However, as discussed above, the time corresponding to thepeak of the emitted light pulse T₁ may not be known or directlymeasurable. Instead, the LIDAR sensor assembly 100 may determine ameasured time of flight between transmission of the firing signal T₀ andthe peak of the received pulse T₂. That is, the measured time of flightequals T₂−T₀. Then, unlike conventional systems, the LIDAR sensorassembly 100 can compute T₁ based on the received pulse from thereference surface. Specifically, using the known distance D to thereference surface and the speed of light, the LIDAR sensor assembly 100can compute the expected time of flight (i.e., the amount of time itshould take the emitted pulse to complete the round trip to thereference surface and back). From this, the LIDAR sensor assembly 100can compute the time corresponding to the peak of the emitted lightpulse T₁. This also allows, the LIDAR sensor assembly 100 to determine afiring latency (i.e., an amount of time from issuance of the firingsignal T₀ to the peak of the emitted light pulse T₁) attributable toperformance characteristics and limitations of the components of theLIDAR systems, such as the capacitive drivers 132 and the light sources104.

In other examples, due to the relatively short distance D to thereference surface (typically a few centimeters or less), the time offlight of the light pulse to the reference surface may be negligible(i.e., T₂−T₁ may be negligible) when compared with time of flight oflight pulses emitted in the detection angle of the LIDAR sensor assembly(i.e., pulses emitted into the surroundings of the LIDAR sensor assemblyto detect objects in the surroundings), which are typically in the rangeof about 1 meter to about 100 meters from the LIDAR sensor assembly. Inthat case, the LIDAR sensor assembly 100 may treat the firing latency tobe a whole period from the signal to fire T₀ to receipt of the peakreturn signal T₂.

In some examples, the LIDAR sensor assembly 100 may determine when thechassis 102 is oriented to emit light toward the reference surface 122based on the return signals (e.g., the shortest return signal receivedduring each revolution may be determined to correspond to the referencesurface). In other examples, a portion of the rotation of the chassis102 during which pulses are emitted toward the reference surface 122 maybe defined as a reference angle, and a rotary encoder coupled to thechassis 102 may be used to indicate when the chassis 102 is oriented toemit light within the reference angle. Return signals received while thechassis 102 is oriented in the reference angle may be determined tocorrespond to the reference surface.

In some examples, the intensity of the reflected light returned from thereference surface may be measured and compared to previous returns todetect changes in performance of the light sources (e.g., determinedegradation, burnout, or malfunction of a light source, storage capacityof a capacitor or other power source, or the like). For instance, if thepeak of the received reference pulse has a magnitude lower than previousreceived reference pulses, or if a sequence of received reference pulsesshows a downward trend of peak values, the LIDAR sensor assembly 100 maydetermine that the light source corresponding to the emitted pulse isburning out, is damaged, is dirty, or is otherwise in need of service.

In some examples, other characteristics of the return pulse, such as theshape of the return pulse (e.g., how Gaussian, how steep/sharp, howwide, etc.), may additionally or alternatively be measured. The shape ofthe return pulse may provide additional information which may be usefulfor calibration of the LIDAR sensor and/or correlation of emitted andreceived signal pulses, for example.

Example LIDAR Sensor Assembly with Integral Reference Surface(s)

FIGS. 3A, 3B, 4A, and 4B illustrate an example LIDAR sensor assembly 300with one or more integral reference surfaces. In particular, FIG. 3A isa perspective view of the example LIDAR sensor assembly 300 with anouter housing omitted for clarity. FIG. 3B is a simplified top view ofthe example LIDAR sensor assembly 300, with a top support rib omitted.FIG. 4A is a perspective view of the example LIDAR sensor assembly 300showing the outer housing. FIG. 4B is a simplified cross sectional viewof the example LIDAR sensor assembly 300, taken along line B-B of FIG.4A.

FIG. 3A illustrates the LIDAR sensor assembly 300 including a stationaryportion 302 and a rotatable assembly 304 coupled to, and rotatablerelative to, the stationary portion 302. The rotatable assembly 304includes an elongated chassis 306 which houses multiple laser lightsources to emit laser light, multiple light sensors, and associatedcircuitry and electronics (e.g., one or more controllers, chargecircuits, capacitive drivers, ADCs, cross correlators, etc.), such asthose shown in FIG. 1. The elongated chassis 306 has a generally frustumshape, which tapers from a top end to a bottom end. The elongatedchassis 306 has an axis of rotation X substantially at a radial centerof the frustum, about which the rotatable assembly 304 rotates. A lensassembly 308 includes a first lens 310 positioned in an optical path ofthe laser light sources, and a second lens 312 positioned in an opticalpath of the light sensors. In this example, the first lens 310 and thesecond lens 312 each constitute less than a full circle such thatportions of the circumferences of the lenses that are closest togetherare truncated so they can be closer together. In other words, centers ofthe first lens 310 and the second lens 312 are less than one diameterapart from each other. In other examples, however, one or both lensesmay be circular, such as the example shown in FIG. 1.

The stationary portion 302 includes an elongated spine 314 which extendssubstantially parallel to the axis of rotation X of the rotatableassembly 304. The spine 314 may include mounting features (e.g., throughholes, connectors, brackets, etc.) to mount the LIDAR sensor assembly300 to a vehicle, machine, or other surface during operation. The spine314 may additionally house electronics and/or provide a routing pathwayto route conductors to transfer power and/or data between the LIDARsensor assembly and a computing device. A pair of support ribs 316extend substantially perpendicularly from the spine 314 and couple tofirst and second ends of the elongated chassis 306. Specifically, afirst support rib 316A extends substantially perpendicularly from thespine 314 and couples to a first (top) end of the chassis 306, and thesecond support rib 316B extends substantially perpendicularly from thespine 314 and couples to a second (bottom) end of the chassis 306. Thesupport ribs 316 are coupled to the chassis 306 by bearings, bushings,or other rotatable connections allowing the chassis 306 to rotaterelative to the support ribs 316 and spine 314. In the illustratedexample, a motor 318 (e.g., an electric motor) is coupled between thechassis 306 and the support rib 316A and configured to apply torque torotate the rotatable assembly 304 about the axis X However, in otherexamples, the motor 318 may be located in other locations. For instance,the motor may be located on an opposite side of the support rib 316Afrom the chassis 306. In other examples, the motor 318 may be locatedremotely from the chassis 306 and torque from the motor 318 may beprovided by a device for transmitting torque, such as, for example, oneor more gears, one or more shafts, one or more belts, and/or one or morechain drives. In some examples, the motor 318 may be located at thesecond (bottom) end of the chassis 306, for example, between the supportrib 316B and the chassis 306, or on the opposite side of the support rib316B from the chassis 306.

Because the spine 314 is opaque and extends substantially parallel tothe axis of rotation X of the rotatable assembly 304, the spine 314obstructs a portion of a scan path of the laser light sources and limitsa detection angle of the LIDAR sensor assembly 300. Typically, a limiteddetection angle for a LIDAR sensor is undesirable. However, the LIDARsensor assembly 300 described in this example can take advantage of thislimited detection angle by using the spine 314, which is located at aknown distance relative to the rotatable assembly 304, as a fixedreference surface in order to calibrate the LIDAR sensor assemblyaccording to the techniques described herein.

FIG. 3B illustrates the limited detection angle of the LIDAR sensorassembly 300. As shown, the detection angle of the LIDAR sensor assembly300 is indicated by the angle θ. Over the angle θ the LIDAR sensorassembly 300 emits laser light into a scene surrounding the LIDAR sensorassembly 300 to detect objects in the scene. However, over the angle α(360-θ) the spine 314 obstructs the laser light. The angle θ that isobstructed by the spine 314 depends on the width of the spine, thedistance of the spine 314 from the axis of rotation X, the spacing ofthe optical axes of the first lens 310 and the second lens 312. In theillustrated example, the angle θ is about 270 degrees. In some examples,the angle θ may be between about 240 degrees and about 300 degrees.However, in other examples, the angle θ may be greater than or less thanthose listed. For instance, in the case of a nose mounted LIDAR sensorassembly θ may be about 180 degrees, while in a top mounted LIDARassembly θ may be 360 degrees (as will be discussed further with respectto FIG. 5).

Within the angle α, the LIDAR sensor assembly 300 may emit one or morepulses of light. For ease of illustration, FIG. 3B illustrates a singleemitted pulse of light shown by the dashed line 320. However, inpractice the LIDAR sensor assembly may fire multiple pulses of lightfrom multiple different light sources as the sensor rotates throughangle α. Due to the relative proximity of the of the spine 314 to therotatable assembly 304, parallax may result in the emitted light pulse320 from being reflected back at an angle that would not ordinarily beincident on the lens 312 and would, therefore, not be detected by thecorresponding light sensor within the chassis 306. In order to avoidthis parallax problem and ensure that a return signal is received foreach light pulse that is emitted within angle α, a light diffuser 322may be disposed on at least a portion of a surface of the spine 314closest to the rotatable assembly 304. The light diffuser 322 may beformed integrally with the spine 314 or may be applied to all or aportion of the spine 314 (e.g., as a cover, sticker, paint, or coating).The diffuser 322 provides substantial internal reflection, such thatwhen hit with light at any location on its surface, the diffuser emitslight from substantially its entire surface, as shown by the dot dashlines 324 in FIG. 3B. The diffuser 322 can comprise any material capableof providing the desired internal reflection such as, for example, aretroreflector, a white or reflective material having a textured surface(e.g., bead blasted glass or acrylic, acid etched glass, etc.), etc.

Thus, when a controller of the LIDAR sensor assembly causes a lightsource to emit a pulse of laser light toward the reference surface(i.e., anywhere within angle α), a signal is received from the lightsensor indicating detection of reflected light corresponding toreflection of the pulse of laser light from the fixed reference surface.Based on this signal indicating detection of the reflected lightcorresponding to reflection of the pulse of laser light from the fixedreference surface and the known distance to the reference surface, thecontroller is able to calibrate the LIDAR sensor assembly 300 to accountfor performance characteristics of the light sources, drivers of thelight sources, and other components of the LIDAR sensor assembly 300.

While omitted from FIGS. 3A and 3B for clarity, the LIDAR sensorassembly 300 may also include an outer housing, such as the one shownand described with reference to FIGS. 4A and 4B below. The outer housingmay include a substantially transparent ring lens through which light isemitted from and received by the LIDAR sensor assembly 300. Theinclusion of the outer housing, including the ring lens, does notsubstantially change the operation of the LIDAR sensor assembly providedabove. In some examples, the ring lens may be made of an antireflectivematerial and/or interior and exterior surfaces of the ring lens may becoated with an antireflective coating in order to minimize the opticaleffects of the ring lens on the light entering and exiting the LIDARsensor assembly 300.

FIG. 4A is a perspective view of the example LIDAR sensor assembly 300,showing an outer housing 400 to cover and protect the rotatable assembly304 and the electronics of the LIDAR sensor assembly 300. The outerhousing 400 includes an opaque cap 402 and main body 404, and asubstantially transparent ring lens 406 interposed between the cap 402and the main body 404. The cap 402 is disposed at and covers the firstsupport rib 316A and the first end (the top) of the rotatable assembly304 of the LIDAR sensor assembly 300. The main body 404 surrounds andencloses the second support rib 316B and the second end (bottom) of therotatable assembly 304. The ring lens 406 encircles the portion of therotatable assembly 304 through which light enters and exits the lensassembly 308. Thus, the ring lens 406 facilitates the passage of lightto and from the LIDAR sensor assembly 300 as the rotatable assembly 304rotates within the outer housing 400. The outer housing 400 encloses therotatable assembly 304 and is coupled to the stationary portion 302 ofthe LIDAR sensor assembly 300. The cap 402 and the main body 404 arecontoured to generally conform to an outer geometry of the rotatableassembly 304 around a majority of its circumference, before curving atan edge closest to the spine 314 to mate with lateral edges of the spine314. Contoured trim pieces 408 may be included to fill a gap between thering lens 406 and the spine 314 and to match the contours of the cap 402and the main body 404. The contoured trim pieces 408 may be opaque ortransparent. One or more O-rings (not shown) may be provided at theinterfaces between the cap 402 and the ring lens 406, and/or between thering lens 406 and the main body 404, in order to prevent dirt andmoisture from entering the outer housing 400. Gaskets and/or sealantsmay be provided between the outer housing 400 and the spine 314 in orderto prevent dirt and moisture from entering the outer housing 400.

FIG. 4B is a simplified cross sectional view of the example LIDAR sensorassembly 300, taken along line B-B of FIG. 4A. FIG. 4B is similar to theexample described with reference to FIG. 3B except that in this case,instead of (or in addition to) the spine 314, the ring lens 406 servesas the fixed reference surface for calibrating the LIDAR sensorassembly. Like the spine 314, the ring lens 406 is fixed a knowndistance from the rotatable assembly 304 and can also serve as areference surface. Just as in FIG. 3B, for ease of illustration a singlepulse of light, shown by the dashed line 320, is emitted from therotatable assembly 304. However, in practice, the LIDAR sensor assemblymay fire multiple pulses of light from multiple different light sources.In the example of FIG. 4B, at least a portion of the pulse of light isreflected by the ring lens 406. In this example, an interior surface ofthe ring lens 406 may not be coated with an antireflective material. Thereflected portion of the light pulse is shown by the dot dash lines. Asshown, there may be multiple internal reflections of the light pulse. Atleast some of the reflected light enters the lens 312 and is received bya light sensor of the LIDAR sensor assembly 300. Upon receipt of thereflected light, the light sensor generates a signal indicatingdetection of the reflected light corresponding to reflection of thepulse of laser light from the fixed reference surface (the ring lens 406in this example). Based on this return signal from the reference surfaceand the known distance to the reference surface, the controller is ableto calibrate the LIDAR sensor assembly 300 to account for performancecharacteristics of the light sources, drivers of the light sources, andother components of the LIDAR sensor assembly 300.

When the ring lens 406 is used as the reference surface, the calibrationoperation is not necessarily limited to a portion of the rotation duringwhich the scan direction of the rotatable assembly 304 is directedtoward the spine 314. Because a portion of each emitted light pulse isreflected by the ring lens 406 and detected by the light sensors, theLIDAR sensor assembly 300 could be calibrated based on any emitted lightpulse emitted at any angle of rotation of the rotatable assembly 304,not necessarily when oriented toward the spine 314. However, in someexamples, it may be beneficial to calibrate the LIDAR sensor assembly300 based on pulses emitted toward the spine 314 since the system neednot be simultaneously determining a distance to an object in thesurroundings (since the distance to the spine is known). Additionally,in some examples, the spine 314 may include an optically black portion410 (or substantially light absorbing portion). The surface of the spine314 may be made optically black by, for example, constructing all or aportion of the spine of an optically black material, or by applying anoptically black cover, sticker, paint, or other coating. By includingthe optically black portion 410, pulses of light incident on theoptically black portion 410 will be absorbed and will not be reflected.Thus, if the LIDAR sensor assembly 300 is calibrated based on pulsesemitted toward the spine 314, the only return will be the reflectionsfrom the ring lens 406. Thus, reduces noise and thereby reduces thecomputational complexity of calibrating the LIDAR sensor assembly 300based on the return from the ring lens 406 as the reference surface.

Additionally, in some examples, the return from the ring lens 406 as thereference surface may be measured during the calibration, and may befiltered out of subsequent distance measurements (i.e., during theportion of the rotation not obstructed by the spine). During operation,the LIDAR sensor assembly 300 receives multiple returns for every lightemission (e.g., one or more reflections from the ring lens 406 as wellas desired returns from actual objects in the surrounding scene). Duringnormal distance measurements, the reflections from the ring lens 406 areextraneous noise that can degrade the accuracy of the LIDAR sensorassembly. However, in examples that employ an optically black portion410 and use the ring lens 406 as a reference surface, the return signalscorresponding to reflections the ring lens 406 can be isolated andfiltered out, thereby eliminating noise from the return signal andfurther improving accuracy of the LIDAR sensor assembly 300.

FIG. 5 is a simplified cross sectional view of another example LIDARsensor assembly 500 that has an unobstructed 360-degree detection angle.FIG. 5 is similar to the example of FIG. 4B, except that the spine 314and at least one of the support ribs 316A and/or 316B is omitted. Inthis example, the LIDAR sensor assembly 500 is supported entirely by anaxle 502 extending from a top or a bottom of the LIDAR sensor assembly500. Thus, the detection angle of the LIDAR sensor assembly 500 isunobstructed and the LIDAR sensor assembly 500 has a full 360-degreedetection angle. The LIDAR sensor assembly 500 may still employ the ringlens 406 as a reference surface for calibration according to thetechniques described above.

Example System of LIDAR Sensors Coupled to Vehicle

FIGS. 6 illustrates an example system 600 including a multiple LIDARsensor assemblies 602A-602F (referred to collectively as “LIDAR sensorassemblies 602”) mounted to a vehicle 604. The vehicle 604 in thisexample is illustrated as being an autonomous passenger vehicle.However, in other examples, LIDAR assemblies can be mounted tonon-passenger vehicles, robots, aircraft, and other vehicles, and may beautonomous, semi-autonomous, or human driven.

FIG. 6 illustrates four corner mounted LIDAR sensor assemblies602A-602D, a top mounted LIDAR sensor assembly 602E, and a nose mountedLIDAR sensor assembly 602F. The corner mounted LIDAR sensor assemblies602A-602D may be the same as or similar to those shown in FIGS. 3A, 3B,4A, and/or 4B, for example, and may have a detection angle θ₁ of atleast about 240 degrees and at most about 270 degrees. The top mountedLIDAR sensor assembly 602E may be the same as or similar to that shownin FIG. 5, for example, and may have a detection angle θ₂ of about 360degrees. The nose mounted LIDAR sensor assembly 602F may be similar tothose shown in FIGS. 3A, 3B, 4A, and/or 4B, for example, and may have adetection angle θ₃ of at least about 160 degrees and at most about 180degrees. In some examples, the system 600 may include all of theillustrated LIDAR sensor assemblies 602, a subset of the LIDAR sensorassemblies (e.g., only the corner mounted LIDAR sensor assemblies602A-602D), or the system 600 may include additional LIDAR sensorassemblies (e.g., a tail mounted LIDAR assembly, side or door mountedLIDAR sensor assemblies, etc.). The LIDAR sensor assemblies 602 may becoupled to one or more body panels or structural members of the vehicle604 or may be formed integrally with the vehicle body itself (e.g., theLIDAR sensor assembly housing may be formed into the contour of afender, hood, bumper, door, roof, or other portion of the vehicle body).

In some examples, a portion of the vehicle body may be within adetection angle of a LIDAR sensor assembly 602. In that case, the LIDARsensor assembly may be fixed relative to the portion of the vehicle and,thus, the portion of the vehicle body may serve as a fixed referencesurface and may be used for calibration of the LIDAR sensor assemblyaccording to the techniques described herein.

FIG. 7 is a side view of an example system 700 including a multipleLIDAR sensor assemblies 702A and 702B (referred to collectively as“LIDAR sensor assemblies 702”) mounted to a vehicle 704 at differentorientations. Specifically, the system 700 includes a first LIDAR sensorassembly 702A mounted at a front corner of the vehicle 704. The firstLIDAR sensor assembly 702A is mounted such that an axis of rotation X ofthe first LIDAR sensor assembly 702A is oriented substantiallyvertically (i.e., normal to the horizon). The first LIDAR sensorassembly 702A is configured such that a pattern of emitted light pulses706A is spanning the horizon, with some pulses being angled above thehorizon and some pulses that are below the horizon. In some examples,the pattern of emitted light pulses may be concentrated around thehorizon with fewer pulses emitted at angles further from the horizon.However, other scan patterns are also contemplated having light pulsesemitted at other angles relative to the horizon.

The second LIDAR sensor assembly 702B is mounted such that an axis ofrotation X of the first LIDAR sensor assembly 702B is offset by angle ηrelative to vertical (i.e., is tilted at an oblique angle from normal tothe horizon). Nevertheless, the second LIDAR sensor assembly 702B isconfigured such that a pattern of emitted light pulses 706B issubstantially the same as that of LIDAR sensor assembly 702A. This maybe achieved, for example, by angling one or more mirrors in the LIDARsensor assembly. However, again, other scan patterns are alsocontemplated having light pulses emitted at other angles relative to thehorizon.

In some examples, different LIDAR sensor assemblies of the vehicle 704may have different scan patterns. For instance, some LIDAR sensorassemblies (e.g., corner mounted LIDAR sensor assemblies) may have scanpatterns centered around the horizon, while one or more other LIDARsensor assemblies (e.g., nose or tail mounted LIDAR sensor assemblies)may have scan patterns oriented below the horizon (e.g., to detectobjects closer to a front of the vehicle). These and other variations ofmounting configurations are contemplated for LIDAR sensor assembliesaccording to this disclosure.

FIG. 7 also illustrates an example computing architecture 708 of thevehicle 704. The computing architecture 708 includes one or more sensorsystems 710. The sensor system(s) 710 include the LIDAR sensorassemblies 702 and may include one or more other sensor systems such as,for example, one or more cameras, radar sensors, microphones, navigationsensors (e.g., GPS, compass, etc.), motion sensors (e.g., inertialsensors, odometers, etc.), and/or environmental sensors (e.g.,temperature sensors, pressure sensors, humidity sensors, etc.). Thesensor system(s) 710 provide input directly to one or more vehiclesystems 712. The vehicle system(s) 712. In some examples, the vehiclesystem(s) 712 may include a vehicle control system to control steering,propulsion, braking, safety systems, and/or communication systems of thevehicle 704. Additionally, in some examples, such as when the vehicle704 is an autonomous vehicle, the vehicle systems may also include alocalizer system to estimate a change in position of the vehicle 704over time, a perception system to perform object detection and/orclassification, and/or a planner system to determine routs and/ortrajectories to use to control the vehicle. Additional details oflocalizer systems, perception systems, and planner systems that areusable can be found in U.S. patent application Ser. No. 15/281,416,filed Sep. 30, 2016, entitled “Estimating Friction Based On Image Data,”which is incorporated herein by reference.

The computing architecture 708 also includes one or more processors 714and memory 716 communicatively coupled with the one or more processors714. The processor(s) 714 may be any suitable processor capable ofexecuting instructions to implement the vehicle system(s) 712. By way ofexample and not limitation, the processor(s) 714 may comprise one ormore Central Processing Units (CPUs), Graphics Processing Units (GPUs),or any other device or portion of a device that processes electronicdata from registers and/or memory to transform that electronic data intoother electronic data that may be stored in registers and/or memory.

Memory 716 is an example of non-transitory computer-readable media.Memory 716 may store an operating system and one or more softwareapplications, instructions, programs, and/or data to implement themethods described herein and the functions attributed to the varioussystems. In various implementations, the memory may be implemented usingany suitable memory technology, such as static random access memory(SRAM), synchronous dynamic RAM (SDRAM), nonvolatile/Flash-type memory,or any other type of memory capable of storing information.

The computing architecture 708 also includes one or more communicationconnections 718 that enable communication by the vehicle with one ormore other local or remote computing devices. The communicationsconnection(s) 718 include physical and/or logical interfaces forconnecting the computing architecture 708 to another computing device ora network. For example, the communications connection(s) 718 may enableWiFi-based communication such as via frequencies defined by the IEEE802.11 standards, short range wireless frequencies such as Bluetooth®,or any suitable wired or wireless communications protocol that enablesthe respective computing device to interface with the other computingdevices.

The architectures, systems, and individual elements described herein mayinclude many other logical, programmatic, and physical components, ofwhich those shown in the accompanying figures are merely examples thatare related to the discussion herein.

Example Process of Calibrating LIDAR Sensor Assemblies

FIG. 8 is a flowchart illustrating an example method 800 of calibratinga LIDAR sensor assembly using a reference surface that is fixed relativeto the LIDAR sensor assembly. The method 800 is described with referenceto the LIDAR sensor assembly of FIG. 1 for convenience and ease ofunderstanding. However, the method 800 is not limited to being performedusing the LIDAR sensor assembly of FIG. 1 and may be implemented usingany of the other LIDAR sensor assemblies and/or systems described inthis application, as well LIDAR sensor assemblies and systems other thanthose described herein. Moreover, the LIDAR sensor assemblies andsystems are not limited to performing the method 800.

At operation 802, rotatable assembly of a LIDAR sensor assembly, such asLIDAR sensor assembly 100 is caused to rotate. This rotation may becaused by a controller (e.g., controller 124 of the LIDAR sensorassembly, a controller of one of vehicle sensor systems 710, etc.). Asthe rotatable assembly rotates, the LIDAR sensor assembly scans adetection angle by emitting laser light pulses from one or more lightsources (e.g., light sources 104) and receiving reflected returnscorresponding to the emitted light pulses by one or more correspondinglight sensors (e.g., light sensors 106). In some examples, operation 802may be initiated upon startup of a vehicle or other machine to which theLIDAR sensor assembly is used.

At operation 804, the controller of the LIDAR sensor assembly or acontroller of a sensor system of a vehicle determines whether tocalibrate the LIDAR sensor assembly. In some examples, the controllermay be configured to calibrate the LIDAR sensor assembly at least onceper revolution of the rotatable assembly. In some examples, thecontroller may be configured to calibrate the LIDAR sensor assemblyevery time a light source emits light toward a reference surface. Insome examples, the controller may be configured to calibrate the LIDARsensor assembly periodically (e.g., every M units of time or number ofrevolutions, where M is any number greater than or equal to 2). In someexamples, the controller may be configured to calibrate the LIDAR sensorassembly responsive to occurrence of a triggering event such as poweringon the LIDAR sensor assembly, a change in temperature, a difference inmeasurements by the LIDAR sensor assembly and another LIDAR sensorassembly, detection of an impact or other force exceeding a normaloperating conditions, or the like. If the controller determines not tocalibrate the LIDAR sensor assembly, the method returns to operation 802to scan the detection angle of the LIDAR sensor assembly. If, atoperation 804, the controller determines to calibrate the LIDAR sensorassembly, the method proceeds to operation 806.

At operation 806, the controller causes a light source of the LIDARsensor assembly to emit light toward a reference surface that is fixedin relation to the LIDAR sensor assembly. The reference surface maycomprise a part of the LIDAR sensor assembly as in the example of FIGS.3A and 4A, a part of a vehicle, machine, or other structure to which theLIDAR sensor assembly is mounted, or any other surface that is fixed ata known distance relative to the LIDAR sensor assembly. At operation808, the controller receives a signal from a light sensor of the LIDARsensor assembly. The signal received from the light sensor indicatesdetection of reflected light corresponding to reflection of the pulse oflaser light from the reference surface. At operation 810, the controllercalibrates the LIDAR sensor assembly based at least in part on thesignal indicating detection of the reflected light corresponding toreflection of the pulse of laser light from the reference surface.

In some examples, the calibration operation 810 includes, at operation812, measuring a time of flight from a firing signal to fire the pulseof laser light from the laser light source to the detection of thereflected light by the light sensor. At operation 814, the controllercompares the time of flight to an expected time of flight for the pulseof laser light to travel a known distance from the laser light source tothe reference surface and back to the light sensor. And, at operation,816, the controller may adjust a distance calculation based at least inpart on the comparison. In some examples, the calibration operation 810may include other adjustments in addition to or instead of theoperations 812-814. For example, the calibration operation 810 mayinclude measuring an intensity of a return signal indicating detectionof the reflected light corresponding to reflection of the pulse of laserlight from the reference surface. The measured intensity may be comparedto previous returns to detect changes in performance of the lightsources (e.g., determine degradation, burnout, or malfunction of a lightsource, storage capacity of a capacitor or other power source, or thelike). In some examples, a drive signal applied to fire the light sourcemay be adjusted (e.g., by adjusting a charge time of one or morecapacitive drivers) to adjust an intensity of subsequent light pulses.

Operations 806-810 are described for a single channel of a LIDAR sensorassembly. For LIDAR sensor assemblies having multiple channels, theoperations 806-810 may be performed for each channel of the LIDAR sensorassembly. Moreover, the method 800 describes the process for calibratinga single LIDAR sensor assembly. In LIDAR systems including multipleLIDAR sensor assemblies, the method 800 may be performed for each of themultiple LIDAR sensor assemblies.

The method 800 is illustrated as a collection of blocks in logical flowgraph, which represents sequences of operations that can be implementedin hardware, software, or a combination thereof In the context ofsoftware, the blocks represent computer-executable instructions storedon one or more computer-readable storage media that, when executed byone or more processors, perform the recited operations. Generally,computer-executable instructions include routines, programs, objects,components, data structures, and the like that perform particularfunctions or implement particular abstract data types. The order inwhich the operations are described is not intended to be construed as alimitation, and any number of the described blocks can be combined inany order and/or in parallel to implement the processes. In someembodiments, one or more blocks of the process may be omitted entirely.Moreover, the method 800 may be combined in whole or in part.

The various techniques described herein may be implemented in thecontext of computer-executable instructions or software, such as programmodules, that are stored in computer-readable storage and executed bythe processor(s) of one or more computers or other devices such as thoseillustrated in the figures. Generally, program modules include routines,programs, objects, components, data structures, etc., and defineoperating logic for performing particular tasks or implement particularabstract data types.

Other architectures may be used to implement the describedfunctionality, and are intended to be within the scope of thisdisclosure. Furthermore, although specific distributions ofresponsibilities are defined above for purposes of discussion, thevarious functions and responsibilities might be distributed and dividedin different ways, depending on circumstances.

Similarly, software may be stored and distributed in various ways andusing different means, and the particular software storage and executionconfigurations described above may be varied in many different ways.Thus, software implementing the techniques described above may bedistributed on various types of computer-readable media, not limited tothe forms of memory that are specifically described.

Example Clauses

A. An example LIDAR sensor assembly comprises:

a stationary portion having a fixed reference surface; a rotatableassembly coupled to, and rotatable relative to, the stationary portion,the rotatable assembly comprising:

-   -   a laser light source to emit laser light; and    -   a light sensor configured to produce a light signal in response        to sensing reflected light corresponding to reflection of the        laser light emitted by the laser light source from the fixed        reference surface; and

a controller communicatively coupled to the laser light source and thelight sensor, the controller being operative to:

transmit a firing signal to the light source to cause the light sourceto emit a pulse of laser light;

receive a signal from the light sensor indicating detection of reflectedlight corresponding to reflection of the pulse of laser light from thefixed reference surface; and

calibrate the LIDAR sensor assembly based at least in part on the signalindicating detection of the reflected light corresponding to thereflection of the pulse of laser light from the fixed reference surface.

B. An example LIDAR sensor assembly according to example A, wherein thecontroller is operative to calibrate the LIDAR sensor assembly by:

measuring a time of flight from the transmission of the firing signal tothe detection of the reflected light by the light sensor; comparing thetime of flight to an expected time of flight for the pulse of laserlight to travel a known distance from the laser light source to thefixed reference surface and back to the light sensor; and adjusting adistance calculation based at least in part on the comparing.

C. An example LIDAR sensor assembly according to example A or example B,wherein the fixed reference surface is substantially opaque and limits adetection angle of the LIDAR sensor assembly.

D. An example LIDAR sensor assembly according to any one of examplesA-C, wherein the fixed reference surface comprises a light diffuser.

E. An example LIDAR sensor assembly according to example A or example B,wherein the fixed reference surface is substantially transparent.

F. An example LIDAR sensor assembly according to any one of examplesA-C, wherein the rotatable assembly comprises an elongated chassishaving an axis of rotation about which the rotatable assembly isrotatable; wherein the stationary portion comprises: a first support ribrotatably coupled to a first end of the elongated chassis, a secondsupport rib rotatably coupled to a second end of the elongated chassisassembly, and an elongated spine extending between and coupled to thefirst support rib and the second support rib; and wherein the elongatedspine serves as the fixed reference surface.

G. An example LIDAR sensor assembly according to example F, wherein theelongated spine comprises a light diffuser disposed on at least aportion of a surface of the elongated spine closest to the rotatableassembly.

H. An example LIDAR sensor assembly according to any one of examples For G, wherein the elongated spine is substantially parallel to the axisof rotation of the elongated chassis.

I. An example LIDAR sensor assembly according to any one of examplesF-H, wherein the elongated spine comprises a mount for mounting theLIDAR sensor assembly to a vehicle.

J. An example LIDAR sensor assembly according to any one of examplesA-C, wherein the stationary portion comprises a housing at leastpartially enclosing the rotatable assembly, the housing including a ringlens at least partially encircling a portion of the rotatable assemblyfrom which the laser light is emitted and by which the reflected lightis received; and wherein the ring lens serves as the fixed referencesurface.

K. An example method of calibrating a LIDAR system comprises:transmitting a firing signal to a laser light source of a LIDAR sensorassembly to cause the laser light source to emit a pulse of laser lighttoward a reference surface fixed in relation to the LIDAR sensorassembly; receiving a signal from a light sensor of the LIDAR sensorassembly, the signal indicating detection of reflected lightcorresponding to reflection of the pulse of laser light from thereference surface; and calibrating the LIDAR sensor assembly based atleast in part on the signal indicating detection of the reflected lightcorresponding to the reflection of the pulse of laser light from thereference surface.

L. A method according to example K, wherein calibrating the LIDAR sensorassembly comprises: measuring a time of flight from the transmitting ofthe firing signal to the detection of the reflected light by the lightsensor; comparing the time of flight to an expected time of flight forthe pulse of laser light to travel a known distance from the laser lightsource to the reference surface and back to the light sensor; andadjusting a distance calculation based at least in part on thecomparing.

M. A method according to one of examples K or L, further comprisingcausing rotation of a rotatable assembly including the laser lightsource and the light sensor about an axis of rotation to scan adetection angle of the LIDAR sensor assembly.

N. A method according to any one of examples K-M, further comprisingrepeating the generating, the receiving, and the calibrating at leastonce per revolution of the rotatable assembly.

O. A method according to any one of examples K-M, wherein thegenerating, the receiving, and the calibrating are performedperiodically.

P. A method according to any one of examples K-M, wherein thegenerating, the receiving, and the calibrating are performed responsiveto a triggering event.

Q. A method according to any one of examples K-P, further comprisingcalibrating one or more additional LIDAR sensor assemblies by, for arespective LIDAR sensor assembly of the one or more additional LIDARsensor assemblies: transmitting a firing signal to a laser light sourceof the respective LIDAR sensor assembly to cause the laser light sourceto emit a pulse of laser light toward a reference surface fixed inrelation to the respective LIDAR sensor assembly; receiving a signalfrom a light sensor of the respective LIDAR sensor assembly indicatingdetection of reflected light corresponding to reflection of the pulse oflaser light from the reference surface fixed in relation to therespective LIDAR sensor assembly; and calibrating the respective LIDARsensor assembly based at least in part on the signal indicatingdetection of the reflected light corresponding to the reflection of thepulse of laser light from the reference surface fixed in relation to therespective LIDAR sensor assembly.

R. An example system comprises:

a vehicle; and

a LIDAR sensor assembly mounted to the vehicle, the LIDAR assemblyincluding:

-   -   a stationary portion;    -   a rotatable assembly coupled to, and rotatable relative to, the        stationary portion, the rotatable assembly comprising:        -   a laser light source to emit laser light; and        -   a light sensor configured to produce a light signal in            response to sensing reflected light corresponding to            reflection of the laser light emitted by the laser light            source from a reference surface that is fixed at a known            distance in relation to the laser light source and light            sensor; and    -   a controller communicatively coupled to the laser light source        and the light sensor, the controller being operative to:        -   transmit a firing signal to the laser light source to cause            the laser light source to emit a pulse of laser light;        -   receive a signal from the light sensor indicating detection            of reflected light corresponding to reflection of the pulse            of laser light from the reference surface; and        -   calibrate the LIDAR sensor assembly based at least in part            on the signal indicating detection of the reflected light            corresponding to the reflection of the pulse of laser light            from the reference surface.

S. A system according to example R, wherein the controller is operativeto calibrate the LIDAR sensor assembly by: measuring a time of flightfrom the transmission of the firing signal to the detection of thereflected light by the light sensor; comparing the time of flight to anexpected time of flight for the pulse of laser light to travel a knowndistance from the laser light source to the reference surface and backto the light sensor; and adjusting a distance calculation based at leastin part on the comparing.

T. A system according to one of example R or example S, wherein thereference surface is substantially opaque and limits a detection angleof the LIDAR sensor assembly.

U. A system according to any one of examples R-T, wherein the referencesurface comprises a light diffuser.

V. A system according to one of example R or example S, wherein thereference surface is substantially transparent.

W. A system according to any one of examples R-V, wherein the referencesurface comprises a portion of the LIDAR sensor assembly.

X. A system according to any one of examples R-V, wherein the referencesurface comprises a portion of the vehicle.

Y. A system according to any one of examples R-X, wherein the vehiclecomprises an autonomous vehicle. 101011 Z. A system according to any oneof examples R-Y, further comprising one or more additional LIDAR sensorassemblies mounted to the vehicle, such that the system includesmultiple LIDAR sensor assemblies mounted to the vehicle.

AA. A system according to example Z, wherein the vehicle obstructs aportion of a detection angle of at least one LIDAR sensor assembly ofthe multiple LIDAR sensor assemblies.

Conclusion

Although the discussion above sets forth example implementations of thedescribed techniques, other architectures may be used to implement thedescribed functionality, and are intended to be within the scope of thisdisclosure. Furthermore, although the subject matter has been describedin language specific to structural features and/or methodological acts,it is to be understood that the subject matter defined in the appendedclaims is not necessarily limited to the specific features or actsdescribed. Rather, the specific features and acts are disclosed asexemplary forms of implementing the claims.

1-20. (canceled)
 21. A method comprising: transmitting a firing signalto a laser light source of a LIDAR sensor assembly to cause the laserlight source to emit light toward a reference surface mechanicallycoupled to the LIDAR sensor assembly, the reference surface being partof a stationary portion of the LIDAR sensor assembly, the stationaryportion comprising: a first support rib and a second support rib; and anelongated spine extending between and coupled to the first support riband the second support rib, the elongated spine serving as the referencesurface; wherein the LIDAR sensor assembly further includes a rotatableassembly mechanically coupled to, and rotatable relative to, thestationary portion, the rotatable assembly comprising: an elongatedchassis having an axis of rotation about which the rotatable assembly isrotatable, wherein the first support rib is rotatably coupled to a firstend of the elongated chassis and the second support rib is rotatablycoupled to a second end of the elongated chassis; receiving a signalfrom a light sensor of the LIDAR sensor assembly; and detecting, basedat least in part on the received signal, reflected light correspondingto reflection of the emitted light from the reference surface.
 22. Themethod of claim 21, further comprising causing rotation of the rotatableassembly about the axis of rotation.
 23. The method of claim 22, furthercomprising repeating the transmitting, the receiving, and the detectingat least once per revolution of the rotatable assembly.
 24. The methodof claim 22, wherein the transmitting, the receiving, and the detectingare performed responsive to a triggering event.
 25. The method of claim21, wherein the elongated spine comprises a mount for mounting the LIDARsensor assembly to a vehicle.
 26. A LIDAR sensor assembly comprising: arotatable assembly comprising: a chassis having an axis of rotationabout which the rotatable assembly is rotatable; a light source to emitlight; a light sensor configured to produce a light signal in responseto sensing reflected light corresponding to reflection of the lightemitted by the light source; and a stationary portion to which therotatable assembly is rotatably coupled, the stationary portion havingan opaque surface disposed in at least a portion of a path of the lightsource; a controller communicatively coupled to the laser light sourceand the light sensor, the controller being operative to: transmit afiring signal to the light source to cause the light source to emitlight; and receive a signal from the light sensor.
 27. The LIDAR sensorassembly of claim 26, wherein the opaque surface comprises a lightdiffuser.
 28. The LIDAR sensor assembly of claim 26, further comprisinga mount for mounting the LIDAR sensor assembly, wherein the opaquesurface comprises at least a portion of the mount.
 29. The LIDAR sensorassembly of claim 26, wherein the stationary portion comprises: a firstsupport rotatably coupled to a first end of the chassis; a secondsupport rotatably coupled to a second end of the chassis; and anelongated member extending between and coupled to the first support andthe second support; and wherein the elongated member serves as theopaque surface.
 30. The LIDAR sensor assembly of claim 29, wherein theelongated spine is substantially parallel to the axis of rotation of thechassis.
 31. The LIDAR sensor assembly of claim 29, wherein theelongated spine comprises a mount for mounting the LIDAR sensor assemblyto a vehicle.
 32. The LIDAR sensor assembly of claim 26, wherein thestationary portion comprises a housing at least partially enclosing therotatable assembly, the housing including a ring lens at least partiallyencircling a portion of the rotatable assembly.
 33. The LIDAR sensorassembly of claim 26, further comprising an outer housing at leastpartially enclosing the rotatable assembly, the outer housingcomprising: a cap; and a main body; and a ring lens interposed betweenthe cap and the main body.
 34. The LIDAR sensor assembly of claim 23,further comprising one or more trim pieces disposed between the ringlens and the opaque surface.
 35. method comprising: transmitting afiring signal to a light source of a LIDAR sensor assembly to cause thelight source to emit light toward a reference surface mechanicallycoupled to the LIDAR sensor assembly, the reference surface being partof a stationary portion of the LIDAR sensor assembly, the stationaryportion comprising: an opaque surface disposed in at least a portion ofa path of the light source; wherein the LIDAR sensor assembly furtherincludes a rotatable assembly mechanically coupled to, and rotatablerelative to, the stationary portion, the rotatable assembly comprising:a chassis having an axis of rotation about which the rotatable assemblyis rotatable; receiving a signal from a light sensor of the LIDAR sensorassembly; and detecting, based at least in part on the received signal,reflected light corresponding to reflection of the emitted light fromthe reference surface.
 36. The method of claim 25, further comprisingcausing rotation of the rotatable assembly about the axis of rotation.37. The method of claim 26, further comprising repeating thetransmitting, the receiving, and the detecting at least once perrevolution of the rotatable assembly.
 38. The method of claim 25,wherein the transmitting, the receiving, and the detecting are performedresponsive to a triggering event.
 39. The method of claim 25, whereinthe chassis comprises a mount for mounting the LIDAR sensor assembly toa vehicle.
 40. The method of claim 25, wherein the reference surfacelimits a detection angle of the LIDAR sensor assembly.